In related art, as an apparatus for supplying oil pressure to a transmission of a motor vehicle, there has been used an apparatus which includes only a main pump driven by an engine serving as a main power supply.
However, in order to implement a start-stop function for allowing an engine to stop when a motor vehicle stops, it is necessary to provide two oil pressure sources such as a conventional main pump and an auxiliary pump driven by an electric motor driven by a battery as a power source so as to ensure supply of oil pressure to a drive system such as the transmission even while the engine stops due to stop of idling. Patent Document 1 describes an oil pressure supply apparatus for a transmission including two oil pressure sources as described above. This oil pressure supply apparatus supplies oil pressure to the transmission, and an auxiliary pump configures an electric pump unit together with an electric motor which drives the auxiliary pump and a motor control apparatus. When oil pressure in a main oil discharge path from the main pump to the transmission is equal to or higher than a predetermined value, the driving of the auxiliary pump is stopped, whereas when the oil pressure in the main oil discharge path is lower than the predetermined value, the auxiliary pump is driven. The oil pressure supplied by the main pump is several tens times larger than oil pressure supplied by the auxiliary pump. Since a measuring range of an oil pressure sensor is set based on the magnitude of the oil pressure supplied by the main pump, using this measuring range for controlling the oil pressure of the auxiliary pump results in an insufficient measuring accuracy, and hence, it becomes difficult to control the oil pressure of the auxiliary pump. In driving of the auxiliary pump, oil pressure equal to or higher than target oil pressure is obtained by driving the electric motor based on a current command value from a host ECU.